Piezoelectric speakers are known. As an example, the piezoelectric speaker in FIG. 8 includes substrate 27 which has a film piezoelectric element 22 adhered to an outer curve thereof. The film piezoelectric element 22 comprises two electrode films 25a and 25b and a high polymer piezoelectric film 24 therebetween. This speaker also has lead wires 26a and 26b connected to the electrode films 25a and 25b to an input electric sound signal. The speaker then can output sound from an inner curve of the substrate 27.
However, the above-mentioned conventional piezoelectric speaker is defective in that it cannot be made planar, because an output sound level thereof is decreased too low to be employed in practical use. The speaker, therefore, has to be shaped to a curve. This restricts its uses to places and ways of installation. For example, it is difficult to hang it on or lean it against walls or pillars.
Further, the speaker involves the problem that if ornamented with a design or pattern for use as poster, the design or pattern can be distorted as a curve, affecting the environment.
As another example there has been proposed a ceramic piezoelectric speaker. This art is advantageous in that the piezoelectric ceramic provides a rather high conversion efficiency of electric power to acoustic power in unit area. However, the ceramic piezoelectric device is defective in that it is difficult to fabricate piezoelectric ceramics of large area. This means that the ceramic piezoelectric device itself cannot provide a high acoustic conversion efficiency, resulting in low acoustic power. To produce higher acoustic power, as shown in FIG. 9, the conventional ceramic piezoelectric device 28 is encased in a resonant cavity 29 the placement and installation of the speaker also is restricted, and cannot readily be hung on or leaned against walls or pillars.